Precise measurement of the $t\bar{t}$ production cross-section and lepton differential distributions in $e\mu$ dilepton events from $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector

The inclusive top quark pair ($t\bar{t}$) cross-section ${\sigma }_{t\bar{t}}$ has been measured in $\sqrt{s}=13$ TeV proton-proton collisions, using 140 fb${}^{-1}$ of data collected by the ATLAS experiment at the Large Hadron Collider. Using events with an opposite-charge $e\mu$ pair and $b$-tagged jets, the cross-section is measured to be: \begin{equation}\nonumber σ_{t\bar{t}} = 829.3 \pm 1.3\,\mathrm{(stat)}\ \pm 8.0\,\mathrm{(syst)}\ \pm 7.3\,\mathrm{(lumi)}\ \pm 1.9\,\mathrm{(beam)}\,\mathrm{pb}, \end{equation} where the uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, the integrated luminosity, and the proton beam energy, giving a total uncertainty of 1.3%. The result is used to determine the top quark pole mass via the dependence of the predicted cross-section on $m_t^{\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{e}}$, giving $m_t^{\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{e}}=172.8_{-1.7}^{+1.5}$ GeV. The same event sample is used to measure absolute and normalised differential cross-sections for the $t\bar{t}\to e\mu \nu \bar{\nu }b\bar{b}$ process as a function of single-lepton and dilepton kinematic variables. Complementary measurements of $e\mu b\bar{b}$ production, treating both $t\bar{t}$ and $Wt$ events as signal, are also provided. Both sets of differential cross-sections are compared to the predictions of various Monte Carlo event generators, demonstrating that the state-of-the-art generators Powheg MiNNLO and Powheg $bb4l$ describe the data better than Powheg hvq. The sensitivity of some of the measured differential distributions to quasi-bound-state formation near the $t\bar{t}$ threshold is investigated in an addendum.